This invention relates to telephone systems and, more specifically, to a selective commanding of modes of data transmittal for protection of communications from data loss. The selection of the degree of protection is particularly useful in cellular mobile radio-telephone systems wherein it is frequently desirable to treat various communications, such as voice, vehicular registration, call event, and various types of control data at differing levels of priority.
A system of interest in understanding cellular mobile radio is disclosed in a U.S. patent application entitled "Cellular Mobile Radio Service Telephone System" of S. O. Goldman et al having Ser. No. 457,155, filed Jan. 11, 1983 and assigned to the assignee hereof, which application is incorporated by reference herein in its entirety.
Cellular radio-telephony is used at numerous sites in this country and overseas. Such a telephone system permits communication between two mobile telephone stations as well as between a mobile station and a fixed station.
Such systems are formed of clusters of cells wherein each cell is assigned to a predetermined set of voice frequency channels. The channels are spaced apart in the frequency spectrum so as to permit simultaneous transmission of many telephone conversations by many stations without interference between communications in the various channels. In order to insure that there is no interference between the assigned frequency channels of one cell and the assigned frequency channels of a contiguous cell, the channels in the contiguous cell are located at different portions of the frequency spectrum than the channels of the one cell. The same frequency channels are repeated at more remote cells, and the power of the signal transmission in any one channel is limited in amplitude so as to become attenuated to a sufficiently low, non-interfering level at the frequency channel of the remote cell.
Cellular mobile radio-telephone systems are described in the literature. One such system referred to as an "Advanced Mobile Phone Service" is described in The Bell System Technical Journal, January 1979, Vol. 58, No. 1, pp 1-269. Multiplexing of the cell site base transceivers and other equipment control links for individual subscriber channels for communication via RF (radio frequency) link is accomplished, preferably, by means of statistical multiplexers. Such multiplexers are described in an article entitled "Controlling Data Communications: Statistical Multiplexer Moves In" by H. J. Hindin in Electronics July 28, 1981, pp 141-148, and in "A Buyers Guide to Today's Volatile Statistical Multiplexers" by J. H. Sharen-Guivel and A. A. Calson in Data Communications, March 1982, pp. 97-126. A switching configuration for a mobile system is disclosed in "A Distributed Switching Approach to Cellular Coverage" by R. E. Pickett in Telecommunications Magazine, February, 1983. A network control system for use in cellular mobile radio-telephony may include the commercially available ITT System 1210 CELLTREX hardware and software.
In the construction of a cellular system, a group of the foregoing cells is clustered about a system switching network which allocates the available frequency channels in any one cell among the various mobile radio-telephones with which communication is desired. Such switching networks provide for the coupling of a telephone conversation of one frequency channel in a first cell with a second frequency channel in a second cell or, alternatively, with a long distance trunk circuit which connects the first cell with a desired cell in another cluster or with a fixed station. In addition, well-known control circuitry is provided for the transmission of command signals to the mobile stations for directing their respective transmissions on the allocated frequency channels.
As a mobile station moves from one cell to the next cell, a hand-off procedure is followed wherein the central switching network commands the mobile station to switch frequency from the channel which was used in the first cell to the frequency of a new channel to be used in the second cell. A characteristic in hand-off decision-making circuitry presently in use is the measurement of the amplitude of or quality of signal transmission with the mobile station. The communication system may include directional antennas at each cell site, the antennas designating specific azimuthal sectors showing generally the position of a mobile station within a cell. The amplitude of or quality of the signal received from a mobile station varies with location of the station in the cell. Thus, the signal strength can serve as an indication that the mobile transmitter is centrally located within a cell, or is located near the boundary of the cell. Thereby, by monitoring the amplitude or quality of such signal transmissions the decision-making circuitry of the hand-off apparatus is able to signal the system switching network at the appropriate time when a hand-off is to be made from one frequency channel to another frequency channel.
It is recognized in telephonic communication that some messages need be transmitted with greater fidelity than others. Protection against errors can be provided by retransmission of a message in the event of an error in the original transmission. An acknowledgement of receipt also protects the sender from the possibility that a message has been missent.
A problem arises in that the foregoing forms of protection are time-consuming and would result, therefore, in reduced rate of transmission if applied generally to all messages. An occasional use of such protection procedures would not significantly reduce the overall data handling capacity of the telephone system. The problem is most significant in mobile telephony because additional data must be transmitted in addition to the usual voice communication such additional data including vehicle identification, channel frequency assignments during hand-off, and other control data as well as digitized data which might be transmitted between subscribers over a voice channel.